Let $E_{/\Bbb Q}$ be a modular elliptic curve, and $p>3$ a good ordinary
or semistable prime. Under mild hypotheses, we prove an exact formula
for the $\mu$-invariant associated to the weight-deformation of the Tate
module of $E$. For example, at ordinary primes in the range $3 < p < 100$,
the result implies the triviality of the $\mu$-invariant of $X_0(11)$.